After going past 3.2k on the rpm meter the car gives a p0300 code. The check engine flashes but goes away after I restart the car. Starting last year I changed the plugs, wires, rotor, cap, and the pcv valve. All oem parts. 4 months ago I upgraded the fuel injectors, changed the pre cat oxygen sensors, fuel filter, fuel pump, and MAF sensor.
I can't think of what could be causing this misfire problem.
Any pointers?

 

I had the same problem... I used a oem part.. Posts in side distributor cap where corroded... Scrapped them off... Put small amount of dielectric grease on posts and problem went away i might not be the best way but it worked for me... ... Might work in your case too... I also know 4.3 vortecs work best with acdelco or delphi caps on them.....

 

Could also be an arcing wire. Check after dark, no lights engine running , look for sparks coming from a spark plug wire. replace as needed